Heartworm in Australian Dogs: Prevention, Testing and Missed Doses
Mosquito season is starting. Here is how heartworm actually spreads, why 'I gave the tablets' isn't always enough, and how to choose between monthly and yearly prevention
Heartworm is spread only by the bite of an infected mosquito. It cannot pass directly between dogs, and a single bite is enough to infect one.
Early infection shows no signs, so waiting for symptoms means waiting until the disease is well advanced. Prevention is far easier than treatment.
Monthly products only work if every dose is given on time. If you forget, ask your vet about testing, because heartworm cannot be detected in blood until about six months after infection.
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- 🦟Spread only by mosquito bites
- 🚫Cannot pass directly from dog to dog
- 🤫No signs in the early stages
- 📅Monthly chew or a yearly vet injection
- 🩸Yearly blood test is recommended
- ⏳Takes about 6 months to show on a test
Heartworm is the disease that sounds like a problem for other people's dogs. Your dog seems perfectly healthy, the tablets are in the pantry somewhere, and you are fairly sure you gave one recently. The trouble is that heartworm gives you no warning. By the time a dog coughs, tires on walks or struggles to breathe, adult worms have been living in the heart and lungs for months.
Spring is when the mosquito population starts to climb, which makes it a good week to check your dog's protection is actually current. This article covers how the disease works, what the prevention options are, and the part most owners get wrong: what to do when a dose is late or missed. It is general information, and your vet is the right person to choose a product and schedule for your dog and your region.
From an infected mosquito is enough
Signs in the early stages of infection
Before infection shows on a blood test
Doses a year for monthly prevention
How Dogs Get Heartworm
There is only one route: the bite of an infected mosquito. A mosquito feeds on an infected animal, picks up immature worms, and passes them to the next dog it bites. Dogs cannot catch it from each other, from the environment or from sharing a bowl, so no amount of caution about other dogs protects yours. The only protection is stopping the immature worms from developing, which is what prevention products do.
Heartworm is found across Australia. It is most common in warm, humid coastal regions, but mosquitoes are present in many areas for much of the year, and Sydney vets note they are around all year. Your local risk is a question for your vet, who knows what is circulating in your area.
Why It Is So Easy to Miss
In the early stages there are simply no signs. As the worms mature and multiply, dogs may develop a cough, become reluctant to exercise and tire quickly. Later signs include pale gums, difficulty breathing and collapse, and severe cases can end in sudden death. Treating an established infection is a long, expensive and risky process compared with a dose of prevention.
See a vet if you notice
A new or persistent cough, reduced stamina on walks, laboured or fast breathing, pale gums, or a dog who collapses. These can have other causes, but with heartworm on the list you do not want to guess.
Monthly or Yearly?
There are two main approaches, and the difference between them is mostly about how much depends on you remembering.
| Monthly chew or tablet | Yearly injection | |
|---|---|---|
| How it works | Given by you every month | Slow-release injection given by your vet |
| What you have to remember | Every dose, on time, all year | One appointment a year |
| If you are a few weeks late | Little or no buffer, and protection may lapse | Designed to cover a delay of a couple of months |
| Often bundled with | Flea, tick and worm cover in combination products | Usually given alongside the annual vaccination |
| Good fit for | Owners who are reliable with a calendar | Owners who struggle with monthly routines, or dogs hard to medicate |
The compliance problem
A figure widely quoted by Australian vet clinics is that around 40 percent of dogs in Australia diagnosed with heartworm were on monthly preventatives at the time. The most likely explanation is missed or late doses rather than the products failing. It is a good reason to set a reminder that cannot be ignored, or to ask your vet whether the yearly injection suits you better.
Testing: When It Matters
A heartworm test is a quick blood test, and there is a wrinkle that catches people out: it can only detect infection about six months after the mosquito bite, because the worms have to mature before the test can see them. That timing shapes the rules.
| Situation | What vets generally advise |
|---|---|
| Dog over 7 months and never on prevention | Test before starting prevention |
| Dog on prevention, annual check-up | Many vets recommend a yearly test to confirm it is working |
| You missed one or more monthly doses | Speak to your vet. Testing straight away can miss a new infection, so they may advise a test months later |
| Puppy under about 7 months | Vets commonly start prevention without a test, since infections take months to show |
If you miss a dose, do not just carry on
Giving the next tablet and hoping is the usual response, but the gap may have left your dog unprotected. Ring your vet, tell them exactly how late you were, and follow their advice on restarting and when to test. That one call is far cheaper than treatment.
Making Monthly Prevention Stick
A System That Does Not Rely on Memory
Pick a fixed date, not 'sometime this week'
The first of the month, or the day you pay the mortgage. Tie it to something you never forget.
Set a repeating reminder with an alarm
A calendar notification you can swipe away is easy to ignore. Use an alarm that needs a tap to confirm.
Mark it off where you will see it
A tick on the fridge calendar, or a note in the same place each month, so a glance tells you whether it was done.
Buy the next supply before this one runs out
Running out is a classic cause of a late dose. Reorder when you open the last packet.
Ask about the yearly injection
If you have missed doses before, an annual injection given at the vaccination visit removes the monthly task altogether.
Products That Help
Prevention products should be chosen with your vet, based on your dog's weight, age and health, and where you live. The two below are practical examples, not a recommendation to bypass that conversation.
NexGard Spectra Chews, Small Dogs 3.6-7.5 kg (3 pack)
A monthly beef-flavoured chew that combines heartworm prevention with flea, paralysis tick, mite and intestinal worm control in one dose, which cuts the number of separate products to remember. The listing is for the small dog weight band, and there are other sizes for larger dogs, so match the pack to your dog's current weight and check with your vet.
Greenies Pill Pockets, Capsule Size (30 count)
If your dog's monthly product is a tablet rather than a chew, or they need any other medication, a pill pocket makes it easy. Drop the tablet in, pinch it closed and hand it over, which beats wrestling with a reluctant dog. A dog who takes their dose happily is a dog who actually gets it.
Heartworm Protection Checklist
- Confirm my dog is on a heartworm prevention product right now
- Work out the date the last dose was given
- Set a repeating alarm for the next dose
- Buy the next supply before this one runs out
- Book a yearly heartworm test with the annual check-up
- Ask my vet whether the yearly injection suits us
- Ring my vet straight away if a dose is missed
- Update the product when my dog moves to a new weight band
What About Repellents?
Reducing mosquito exposure, such as emptying standing water and keeping dogs inside around dusk when mosquitoes are most active, is sensible, but it does not replace prevention. One bite is enough, and you cannot fully control where your dog goes. Do not use insect repellents made for people on your dog, since some ingredients are harmful if licked. Ask your vet about anything applied to the skin.
Check the date of your dog's last dose today. If it is more than a month ago, or you are not sure, that is the call to make this week.
Frequently asked questions
Keep Reading
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